A baby paddlefish enters the LRT looking like a baby shark

Earlier the primitive paddlefish
(Polyodonentered the large reptile tree (LRT, 1634+ taxa) as the basalmost bony fish, distinct from and not related to traditional chondrostean relatives like the sturgeon (Psuedoschaphorhynchus), the bichir (Polypterus), and the extinct ‘chondrostean’ Chondrosteus.

Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.

Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.

Today, a baby paddlefish,
lacking a long paddle bill (Fig. 2), enters the LRT. Why? Because the paddle bill of the adult stands out as an autapomorphic trait in the cladogram, and I’m looking for plesiomorphic transitional taxa that link clades together.  The short-snouted baby paddlefish (Fig. 2), looking just like a baby shark, but with an operculum, had the potential to do exactly that.

Figure 2. A shark-like juvenile paddlefish (Polyodon) has teeth and lacks a paddle-snout. Compare to the adult in figure 1.

Figure 2. A shark-like juvenile paddlefish (Polyodon) has teeth and lacks a paddle-snout. Compare to the adult in figure 1. Legnth = 2.9cm. or slightly longer than one inch. Images from Grande and Bemis 1991, The pterygoid appears here for the first time.

Despite the lack of an elongate rostrum
and the score changes that brings, the baby and adult Polyodon nest together in the LRT.

Figure 2. Falcatus traced with DGS methods with reconstructed freehand image applied from xxx.

Figure 3. Falcatus nests at the base of the shark clade, not far from baby Polyodon. Note the same underslung jaw loosely connected to the cranium + rostrum.

If ontogeny recapitulates phylogeny
in Polyodon, then the baby provides insight into the plesiomorphic morphology of a basalmost bony fish taxon, not far from a basalmost and transitional shark-clade taxa. That dichotomy likely extends back to the Late Silurian or Early Devonian. Thus, this long-sought and previously elusive mystery taxon might be best represented by a baby paddlefish. That means it was  under our nose all along!

BTW
in the ‘old’ days, Polyodon used to be called the ‘paddle-bill catfish.’ Not sure when the common name change took place.


References
Grande L and Bemis WE 1991. Osteology and phylogenetic relationships of fossil and Recent paddlefishes (Polyodontidae) with comments on the interrelationships of Acipenseriformes. Society of Vertebrate Paleontology Memoir 1. Journal of Vertebrate Paleontology 11, Supplement to Number 1. 121pp.
Walbaum J 1792. Petri Artedi renovati. Part 3. Petri Artedi sueci genera Piscium in quibus systema totum ichthyologiae proponitur cum classibus, ordinibus, generum characteribus, specierum diffentiis, observationibus plumiris. Redactis Speciebus 2. Ichthyologiae, III: 723.

wiki/Polyodon

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